1. Field of the Invention
The present invention relates to the radiosynchronization of timekeepers which can be set to the correct time automatically by a radio data system (RDS) radio broadcast transmitter.
2. Description of the Related Art
Radio stations operating in the frequency modulation (FM) band and in accordance with the RDS standard transmit a time signal containing the time and the date coded in a CT portion of a frame transmitted with the audio signal of the station.
FIG. 1 of the appended drawings shows the standardized baseband spectrum transmitted by stations of the above type, which frequency modulates a carrier P whose frequency is specific to the transmitter station. It can be seen that the spectrum includes a first monophonic band B1 from 0 to 15 kHz and which contains all of the right and left audio channel signals to be transmitted. A pilot frequency FP of 19 kHz is used to decode the stereo information, while a second stereophonic band B2 from 23 to 53 kHz contains the difference between the right and left audio signals. Finally, a third band B3 contains the RDS data. This band of the spectrum is centered on 57 kHz with a bandwidth from 54.5 to 59.5 kHz.
The patent GB 2 238 438 discloses a timekeeper equipped with an RDS radio reception device comprising:                a timebase,        means for displaying time data supplied by said timebase, and        means for correcting said time data,        and in which said RDS radio reception device comprises:                    means for delivering RDS data derived from an RDS spectrum received on a high-frequency carrier; and            control means which, on the basis of the delivered RDS data, control said correction means to ensure time setting of the timekeeper.                        
The RDS radio reception device described in the patent previously cited uses the timing data from the RDS frame contained in band B3 of the FIG. 1 spectrum for ensuring the radiosynchronization of a clock provided as a timekeeper in the receiver. However, as the reception device is primarily intended for listening to audio broadcasts by stations to which it can be tuned, it must include reception and sound reproduction circuits that require a relatively large quantity of energy to function.
The reception device must therefore have a high-capacity energy source of which only a very small portion is used for radiosynchronization. In the example provided in the patent in question, which relates to a radio receiver for automobile vehicles, this kind of supply is naturally readily available in the form of the onboard power supply network of the vehicle, with the result that the quantity of energy required for radiosynchronization is not a problem for the designer.
The same would apply in the case of an RDS receiver taking the form of the usual kind of radio with radiosynchronization of a built-in clock, as the dimensions of this kind of radio can accommodate batteries of sufficient capacity to power all of the circuits of the receiver, including the radiosynchronization circuits, for a time period that is acceptable for a user. Such radios can incidentally also be powered by the mains or by a vehicle power supply network.
In the field of clocks and watches, the energy storage capacity available in a timekeeper such as a wristwatch is an ongoing problem that designers attempt as much as possible to solve by minimizing the consumption of all the components of the timekeeper to give it the longest possible battery life from a battery whose size is compatible with that of the timekeeper. Transposing the concept disclosed in the patent previously cited to a timekeeper that is worn by the user, with the aim of radiosynchronizing it using RDS data, thus runs into the problem of supplying electrical energy, as a timekeeper such as a wristwatch can obviously not accommodate an energy source of sufficient capacity for the combination of an RDS HF radio receiver and radiosynchronized clock circuits to be able to operate for a reasonable time period.
A typical wristwatch battery operates at a voltage of 1.3 V and provides a maximum current of the order of only 1 mA, preferably with a battery life of about a year or even more.
Also known in the art are radiosynchronized watches including a radio receiver tuned to a station transmitting a time signal on a long-wave carrier, typically at a frequency from 40 to 80 kHz. These stations are exclusively dedicated to radiosynchronization, and because of their transmit frequency and power, they cover a territory encompassing more than one time zone. The time to which the watch equipped with suitable reception means is synchronized therefore does not necessarily correspond to the time zone in which the user of the watch is located. Also, the consumption associated with the radiosynchronization function in a watch of this kind is relatively low and in any case compatible with a normal period of use of the energy source of the watch. The reason for this is that the low radio frequency carrying the synchronization information allows the use of means whose power consumption is low. Thus this prior art concept cannot provide a satisfactory solution to the problem of producing a timekeeper that is worn by the user and is radiosynchronized by transmissions from HF radio stations transmitting time data in accordance with the RDS standard.